Fill a large bowl with ice water. Bring a large pot of salted water to a boil. Add the fava beans and cook 30 seconds. Remove with a slotted spoon and transfer to the ice water; let cool about 5 minutes (keep the pot of water boiling). Drain the beans, then peel off their skins (they should slip right off). Add the pasta to the boiling water and cook as the label directs. Reserve about 1 1/2 cups cooking water, then drain the pasta.
Heat the olive oil, garlic, mint sprigs, parsley sprigs and all but 2 tablespoons of the scallions in a large skillet over medium heat. Cook, stirring, until the scallions are soft, 1 to 2 minutes. Add the favas and 1 cup of the reserved cooking water. Bring to a simmer and cook until the favas are tender and the liquid is slightly reduced, 8 to 10 minutes; discard the garlic and herb sprigs.
Add the pasta to the skillet and cook, tossing until it absorbs some of the sauce, about 3 minutes. Stir in the butter, chopped mint and parsley, reserved scallions, and salt and pepper to taste. Add more cooking water to loosen, if needed. Serve warm or at room temperature.
Tip: If you can't find fresh fava beans, use frozen shelled and skinned ones and blanch as directed.